1. Field of the Invention
The present invention relates to a cover structure for an air bag device used in a vehicle such as an automobile, more specifically to a cover structure in which a portion of a cover is ruptured to form an opening for exposing an air bag when the air bag is inflated.
2. Description of the Prior Art
Conventionally, there is known a cover structure having a rupturable (tearable) cover as an air bag device, which is provided in an instrument panel installed in a vehicle such as an automobile, as disclosed in Japanese Patent Laid-Open 2001-219805 or Japanese Patent Laid-Open 2000-168401.
In the conventional cover structure, when the air bag is inflated, a rupturable groove formed on a back surface of the instrument panel 300 as the cover, opposite to the air bag device is ruptured and opened to expose an air bag in the air bag device, as shown in FIG. 13.
A portion surrounded by the rupturable groove is formed as a lid 302.
The instrument panel 300 includes a first member 303 having an upper plan or plane-like portion 303b, a lower second member 304 having a vertical portion connected with the first member at a perpendicular angle.
Usually, a backward end 303a of the first member and a forward end 304a of the second member are connected by use of a curved part 305 having a small curvature.
The rupturable groove 301 includes a first groove part 301a extending rightward and leftward, and second groove parts 301b, 301b disposed in right and left sides of the lid for extending forwardly and backwardly of the vehicle. Corners 301c, 301c for connecting the first and second groove parts are disposed in the curved part 305 for connecting the first and second members. If the curved part 305 is formed by the same thickness as the first and second members, a stiffness higher than that of the plane-like portion 303b of the first member 303 due to the configuration of curve and a small area.
Accordingly, since a deformation in a direction of width of the vehicle in the plane-like portion 303b is difficult to generate, a shearing force acts on the instrument panel in a vertical direction to a plane in which the rupturable groove 301 is formed.
On the contrary, a shearing force acts on a region 308 (hatching portion in FIG. 16) of the instrument panel in the rightward and leftward directions at the vicinity of a border line 306 between the first member 303 and curved part 305 since the deformation of curved part is small as shown in FIG. 15.
Especially, as in the conventional instrument panel 300, if the rupture of the rupturable groove is carried out simultaneously or according to the sequence from No. 1 to No. 5 shown in FIG. 15, a direction of shearing force 309 acting on the rupturable groove changes in a direction shown in the arrow in FIG. 15 on the border of the border line 306.
Therefore, as shown in FIGS. 16 and 17, if a portion of the first member adjacent to the rupturable groove 301 is fixed by means of a fixed point 307, a crack 310 (see FIG. 17) tends to generate on the surface of the first member in rightward and leftward directions of the instrument panel. When the lid 302 is opened, a peeling part 311 tends to be formed on the surface of the first member as shown in FIG. 18. Such crack and peeling are easy to generate at a lower temperature (for example, minus 40° C.).
For this reason, if the rupturable groove is easy to rupture by thinning a thickness of the cover in the rupturable groove, at the time of forming the instrument panel, the forming of instrument panel is ineffectively since flowing of liquid synthetic resin is not good in the curbed part 305, and the formed instrument panel is subjected to generation of an upwardly and downwardly repeated resilient deformation in which the instrument panel deforms inwardly of the instrument panel and generates a depression when pressing the instrument panel from the above side.
An improvement is requested in the aforementioned conventional structure.